1. Field of the Disclosure
The disclosure relates generally to fiber optics, and in particular, to a polarization beam splitter/combiner featuring an integrated optical isolator.
2. The Prior Art
Fiber optical networks are becoming increasingly faster and more complex. For example, networks compliant with the OC48 standard for synchronous optical networks (capable of a 2.5 Gb/s data rate) are being replaced by newer networks compliant with OC192 (10 Gb/s). Networks compliant with OC768 (40 Gb/s) networks are already on the horizon. At the same time, research is underway to transmit more and more channels down a single fiber through the use of dense wavelength division multiplexing (DWDM) technology. Eighty-channel systems are currently being deployed; it is anticipated that network density will increase in the future.
Key to this expansion are technologies such as thin film deposition and diffraction gratings which allow optical components to be manufactured in increasingly smaller packages. As optical networks continue to carry more channels at faster rates, component size is becoming a key limiting factor.
Central to any optical network are optical amplifiers. Optical amplifiers such as Raman and erbium-doped fiber amplifiers (EDFA) are responsible for amplifying and transmitting optical signals over long distances.
FIG. 1 shows a prior art operational block diagram of a typical Raman pump model 100. The Raman pump 100 is formed using several discrete components, including four isolators 110, 112, 114, and 116; and two polarization beam combiners (PBC) 118 and 120 in addition to pump lasers (not shown).
In operation, two light sources 102 and 104 feed the two isolators 110 and 112, respectively. The output of the isolators 110 and 112 are fed to PBC 118, where the two signal are combined into a single signal. Likewise, two light sources 106 and 108 feed the isolators 114 and 116, respectively. The output of isolators 114 and 116 feed PBC 120, where the two signals are combined into one signal. The two signals from the PBCs 118 and 120 are then multiplexed and output by WDM 122.
As is appreciated by those skilled in the art, isolators and PBCs are essential components of any optical amplification system. Currently, optical amplifiers must separately employ isolators and PBCs as discrete components. As the complexity of optical networks continues to grow, utilizing discrete components has certain disadvantages. For example, discrete components take up space and are expensive. Furthermore, discrete components must be optically coupled, which may lead to performance degradation.
An integrated optical polarization beam splitter/combiner and isolator (IPBC) is disclosed. In one aspect, a disclosed IPBC may comprise a first birefringent crystal optically configured to receive two rays incident at an angle xcex3; a rotator configured to rotate the two rays received from the first wedge; a second birefringent crystal positioned to receive the two rays from the rotator; and wherein the integrated optical polarization beam splitter/combiner and isolator is configured to combine the two rays in a forward direction, and isolate the two rays in a reverse direction.
In another aspect of a disclosed IPBC, the first and second birefringent crystals may comprise the same material, and have the same wedge angle xcex8.
In a further aspect of a disclosed IPBC, the relationship between the wedge angle xcex8 and the angle xcex3 may be defined as:
xcex3=2xc2x7arc Sin [(noxe2x88x92ne)xc2x7tan xcex8]. 
In yet a further aspect of a disclosed IPBC, the crystals may have optic axes which are 45xc2x0 apart. Furthermore, the two rays may have orthogonal polarizations, and may be combined interior to the second crystal, and exit the second crystal as a third ray.
In a further aspect of a disclosed IPBC, an incoming beam port may be employed for launching the two rays through a lens into the first crystal. The incoming beam port may comprise a plurality of PM fibers, the PM fibers each having corresponding principal axes; the plurality of PM fibers disposed together as a grouping, the grouping having corresponding secondary axes; and whereby each the plurality of PM fibers is aligned such that the corresponding principal axes of each the plurality of the PM fiber and the secondary axes of the grouping intersect at a predetermined angle.
Another aspect of a IPBC is disclosed, which may comprise a first birefringent means for receiving and refracting a first ray and a second ray incident at an angle xcex3 such that the first ray comprises an e-ray with respect to the first wedge, and the second ray comprises an o-ray with respect to the first wedge; rotating means for rotating the two rays received from the first wedge; second birefringent means for receiving and refracting the first and second rays from the rotator such that the first ray comprises an o-ray with respect to the second wedge, and the second ray comprises an e-ray with respect to the second wedge; and wherein the second crystal is optically configured to combine the first and second rays in a forward direction, and the first crystal is optically configured to diverge the first and second rays in a reverse direction.
A method for combining light in a forward direction and isolating light in a reverse direction is disclosed. In one aspect, the method may comprise refracting a first ray and a second ray incident at an angle xcex3 such that the first ray comprises an e-ray with respect to the first wedge, and the second ray comprises an o-ray with respect to the first wedge; rotating the two rays received from the first wedge; and refracting the first and second rays from the rotator such that the first ray comprises an o-ray with respect to the second wedge, and the second ray comprises an e-ray with respect to the second wedge.